The present invention relates to polymer blends comprising at least one (meth)acrylate polymer and at least one amorphous propylene-derived polymer that exhibit pressure sensitive adhesive properties. The pressure sensitive adhesives are useful in preparing a wide variety of articles.
Pressure sensitive adhesive (PSA) compositions are well known to those of ordinary skill in the art to possess properties including the following: (1) aggressive and permanent tack, (2) adherence with no more than finger pressure, (3) sufficient ability to hold onto an adherend, and (4) sufficient cohesive strength. Materials that have been found to function well as PSAs are polymers designed and formulated to exhibit the requisite viscoelastic properties resulting in a desired balance of tack, peel adhesion, and shear holding power. Obtaining the proper balance of properties is not a simple process.
The most commonly used polymers for preparing PSAs are natural rubber-, synthetic rubber- (e.g., styrene/butadiene copolymers (SBR) and styrene/isoprene/styrene (SIS) block copolymers), and various (meth)acrylate- (e.g., acrylate and methacrylate) based polymers. Of these, (meth)acrylate-based polymer PSAs have evolved as a preferred class of PSA due to their optical clarity, permanence of properties over time, and versatility of adhesion levels, to name just a few of the their benefits. It is known to prepare PSAs comprising mixtures of certain (meth)acrylate-based polymers with certain other types of polymers.
European Patent Application No. 0 254 002 (Sumitomo Chemical Co. Ltd.) describes PSAs comprising at least one elastomer (e.g., natural rubber, styrene-butadiene rubber, and acrylic rubber), at least one tackifier, and an ethylene-propylene copolymer having such a low molecular weight that the intrinsic viscosity is not more than 0.5. The ethylene-propylene copolymer is obtainable by oxidative degradation of a corresponding ethylene-propylene copolymer containing 30-60 weight % of propylene. The amount of ethylene-propylene copolymer is in the range of 5-40 parts by weight based on 100 parts by weight of the elastomer. It is taught that the PSA therein is ordinarily dissolved in toluene or the like, coated on a substrate, and then dried to remove the solvent.
U.S. Pat. No. 5,202,361 (Zimmerman et al.) teaches another approach to preparing PSAs using certain (meth)acrylate polymers in combination with certain alpha-olefin polymers. Specifically, the alpha-olefins have a glass transition temperature (Tg) of about xe2x88x9270xc2x0 C. to about xe2x88x9210xc2x0 C. and a weight average molecular weight of about 25,000 to about 5,000,000. Furthermore, at least 60 mole % of the olefin monomers used to prepare the alpha-olefin polymers have 6 to 18 carbon atoms. The PSAs purportedly have good adhesion to both low and high energy surfaces. It is taught that the alpha-olefin polymer is dissolved in a mixture of free-radically polymerizable monomers and a photoinitiator/crosslinker. The liquid composition is then coated on a substrate and cured by irradiating the composition using ultraviolet radiation.
PCT Publication No. WO 97/23,577 (Minnesota Mining and Manufacturing Co.) describes blended PSAs that include at least two components. The first component is a PSA. Among useful PSAs described therein are acrylics and poly alpha-olefins. The second component is a thermoplastic material or elastomer. For example, thermoplastic materials useful in the invention include isotactic polypropylene and ethylene/propylene copolymers. It is also taught that useful thermoplastic materials are essentially immiscible in the PSA component at use temperatures. The Abstract describes the blends therein, which are melt processable, as having a substantially continuous domain (generally the PSA) and a substantially fibrillous to schistose domain (generally the thermoplastic material). Tackifiers may be added.
PCT Publication No. WO 96/25,469 (Minnesota Mining and Manufacturing Co.) describes PSAs that are a blend of about 5 to 95 weight percent of an acrylic PSA and about 5 to about 95 weight percent of a thermoplastic elastomeric copolymer. The thermoplastic elastomeric materials are materials that contain at least two segments, i.e., a hard segment and a soft segment. Useful thermoplastic elastomeric materials include styrene-(ethylene-propylene) block copolymers, polyolefin-based thermoplastic elastomeric materials represented by the formula xe2x80x94(CH2CHR)x, where R is an alkyl group containing 2 to 10 carbon atoms, and polyolefins based on metallocene catalysis, such as an ethylene/1-octene copolymer. The blends are melt processable.
Ways to effectively adhere to low surface energy materials is a challenge that those of ordinary skill in the art are attempting to overcome. Many times improvements in adherence to low surface energy substrates compromises adherence to higher surface energy substrates or compromises shear strength of the adhesive. As such, farther adhesives for adequately adhering to low surface energy surfaces, especially those adhesives that perform without comprising adherence to high surface energy substrates, are desired. Similarly, adhesives with improved adherence to high surface energy substrates are beneficial.
It is also desired that any such new adhesives will allow for broad formulation latitude and tailorability for particular applications. For example, in some applications it is desirable to have a hot-melt processable composition, as opposed to those compositions that are coated on a substrate and subsequently dried or cured. It is also preferred to use (meth)acrylate polymers in such adhesives, due to their desirable properties. The present invention addresses these motivating factors.
PSA blends of the invention are particularly useful for adhering to both relatively high and low surface energy materials. PSA blends of the invention are capable of providing adequate or improved peel adhesion to such substrates. Surprisingly, in preferred embodiments of the invention, peel adhesion to low surface energy substrates, such as polypropylene, is enhanced as compared to peel adhesion of the (meth)acrylate polymer without the propylene-derived polymer to low surface energy substrates or peel adhesion of the propylene-derived polymer without the (meth)acrylate polymer to low surface energy substrates. This enhancement is even possible in some embodiments without causing detrimental effects in peel adhesion to high surface energy substrates. According to other aspects of the invention, peel adhesion to high surface energy substrates, such as glass, is enhanced as compared to peel adhesion of the (meth)acrylate polymer without the propylene-derived polymer to high surface energy substrates or peel adhesion of the propylene-derived polymer without the (meth)acrylate polymer to high surface energy substrates. Useful shear strengths are also realizable using the blends of this invention.
Certain embodiments of the invention provide substrates with the pressure sensitive adhesive composition at least partially applied thereon. Other embodiments of the invention provide fibers and microfiber webs comprising the pressure sensitive adhesive composition.
Pressure sensitive adhesive (PSA) blends of the present invention comprise at least one (meth)acrylate polymer and at least one propylene-derived polymer. Terms used throughout to assist in describing the invention are defined in turn below.
xe2x80x9cPolymerxe2x80x9d refers to macromolecular materials having at least five repeating monomeric units, which may or may not be the same. The term xe2x80x9cpolymerxe2x80x9d, as used herein, encompasses homopolymers and copolymers. Copolymers of the invention refer to those polymers derived from at least two chemically different monomers. Included within the definition of copolymers are traditional copolymers derived from at least five monomers, which include only two chemically different types of monomers, as well as terpolymers, which include at least three chemically different types of monomers, etc.
In general, a polymer can include more than one type of steric structure throughout its chain length. For example, polymers can include crystalline, stereoregular isotactic and syndiotactic structures, as well as amorphous, atactic structures, or combinations thereof The steric structure of a polymer can be determined using any suitable method. For example, carbon- 13 Nuclear Magnetic Resonance can be used to determine the steric structure (i.e., tacticity) of a polymer.
xe2x80x9cStereoregularxe2x80x9d structures, as defined by Hawley""s Condensed Chemical Dictionary (12th Edition), are those whose molecular structure has a definite spatial arrangement, rather than the random and varying arrangement that characterizes an amorphous polymer. Stereoregular structures include isotactic and syndiotactic structures.
xe2x80x9cIsotacticxe2x80x9d structures, as defined by Hawley""s Condensed Chemical Dictionary (12th Edition), are those whose structure is such that groups of atoms that are not part of the backbone structure are located either all above, or all below, atoms in the backbone chain, when the latter are all in one plane.
xe2x80x9cSyndiotacticxe2x80x9d structures, as defined by Hawley""s Condensed Chemical Dictionary (12th Edition), are those whose structure is such that groups of atoms that are not part of the backbone structure are located in some symmetrical and recurring fashion above and below the atoms in the backbone chain, when the latter are all in one plane.
xe2x80x9cAtacticxe2x80x9d structures, as defined by Hawley""s Condensed Chemical Dictionary (12th Edition), are those whose structure is such that groups of atoms are arranged randomly above and below the backbone chain of atoms, when the latter are all in one plane.
The xe2x80x9cStereoregular Index (S.I.)xe2x80x9d of a polymer is defined as follows: In a perfectly atactic polymer, two homotactic triads, mm and rr, are present in equal amounts (25% each). As the polymer becomes increasingly stereoregular, the relative amounts of mm and rr change so that one increases to be greater than the other. S.I. is the ratio of the larger of mm or rr to the smaller of mm or rr and is always positive and greater than 1. S.I. expresses, in a numerical way, how the steric structure of a polymer shifts away from 1.0 for a random, atactic polymer to larger values characteristic of more stereoregular polymers.
xe2x80x9cNon-stereoregularxe2x80x9d polymers are generally mostly atactic or mostly semi-syndiotactic polymers, rather than mostly isotactic or mostly syndiotactic.
xe2x80x9cSemi-syndiotacticxe2x80x9d polymers are those having structures between mostly syndiotactic polymers and mostly atactic polymers.
In one embodiment, non-stereoregular polymers of the invention have an S.I. of 1 to about 10. In another embodiment, non-stereoregular polymers of the invention have an S.I. of 1 to about 7. In still a further embodiment, non-stereoregular polymers of the invention have an S.I. of 1.5 to about 7. In yet another embodiment, non-stereoregular polymers of the invention have an S.I. of 1 to about 1.1.
xe2x80x9cAmorphousxe2x80x9d polymers are those polymers that are hexane soluble at room temperature. Recognize that such materials may have a small degree of crystallinity, which is detectable, for example, using x-ray or thermal analysis. Amorphous polymers lack a well-defined melting point when measured by Differential Scanning Calorimetry (DSC). Particularly preferred are those amorphous polymers that are non-stereoregular (e.g., mostly atactic or mostly semi-syndiotactic).
xe2x80x9cHot melt processablexe2x80x9d refers to those adhesives having a sufficient viscosity upon softening, such that the adhesives can be hot melt processed (e.g., applied to a substrate). It is not necessary for the adhesives to actually melt at the processing temperature, but rather it must soften to the point that it can be made to flow at the processing pressure.
Polymers that have less stereoregularity have been found to be preferred for processing and preparing PSAs of the invention, such as for example, by hot-melt processing. As such, hot-melt processable adhesives are enabled by the present invention.
Hot melt processable adhesives advantageously reduce or eliminate the use of organic solvents in adhesives and their processing. Hot melt processable adhesive systems are essentially 100% solid systems. Usually, such systems have no more than about 5% organic solvents or water, more typically no more than about 3% organic solvents or water. Most typically, such systems are free of organic solvents and water. Advantageously, by reducing the use of organic solvents, special handling concerns associated therewith are also reduced. Furthermore, hot melt processable adhesive systems advantageously do not require a separate processing step after applying the composition to a substrate. Also, in some applications, particularly melt-blown microfiber applications, the adhesive composition must be hot-melt processable.
Another advantage to using polymers that have less stereoregularity is that materials that are highly isotactic tend to be opaque, while those that are less stereoregular tend to be more transparent. The clarity (i.e., transparency) of materials with low stereoregularity makes them preferred for use in applications where clarity of the adhesive is important. Such applications include, for example, bonding of glass and transparent plastics.
One advantage of utilizing blends of the invention is the greater formulation latitude that they provide. That is, changes in a wide variety of physical properties of films comprising the blends can be effectuated, for example, by varying the ratio of individual polymers in the blends. Furthermore, cost effectiveness is another advantage of utilizing blends. For example, less expensive polymers can be blended with more expensive polymers. In that way, the less expensive polymers can act as an xe2x80x9cextenderxe2x80x9d for the more expensive polymers. Also, using blends can provide advantageous synergistic effects, wherein, for a certain application, the blend can perform substantially better than either polymer by itself for the same application.
PSA blends of the invention are particularly useful for adhering to both relatively high and low surface energy materials. PSA blends of the invention are capable of providing adequate or improved peel adhesion to such substrates.
Surprisingly, in preferred embodiments of the invention, peel adhesion to low surface energy substrates, such as polypropylene, is enhanced as compared to peel adhesion of the (meth)acrylate polymer without the propylene-derived polymer to low surface energy substrates or peel adhesion of the propylene-derived polymer without the (meth)acrylate polymer to low surface energy substrates. This enhancement is even possible in some embodiments without causing detrimental effects in peel adhesion to high surface energy substrates.
According to other aspects of the invention, peel adhesion to high surface energy substrates, such as glass, is enhanced as compared to peel adhesion of the (meth)acrylate polymer without the propylene-derived polymer to high surface energy substrates or peel adhesion of the propylene-derived polymer without the (meth)acrylate polymer to high surface energy substrates. Useful shear strengths are also realizable using the blends of this invention.
Preferably at least one of the polymers in the blend is a PSA. However, more than one polymer in the blend may be a PSA. Many polymers are inherently tacky, i.e., the polymers do not require addition of a tackifier to render the composition pressure sensitive. Examples of such inherently tacky polymers include many (meth)acrylate (i.e., methacrylate and acrylate) polymers. However, a polymer may also be made pressure sensitive by addition of a tackifier to the polymer. Whether a polymer is inherently a PSA or requires addition of a tackifier, it is preferred that at least one of the polymers in the blend is a PSA.
(Meth)Acrylate Polymer
Any suitable (meth)acrylate (i.e. acrylate or methacrylate) polymer can be used in blends of the invention. (Meth)acrylate polymers are those derived from at least one (meth)acrylate monomer. (Meth)acrylate polymers may also be derived from, for example, other ethylenically unsaturated monomers and/or acidic monomers and/or the meth)acrylate polymers may also be grafted with a reinforcing polymeric moiety. Specific examples of preferred (meth)acrylate polymers are described in the Examples section, infra.
Particularly preferred (meth)acrylate monomers include (meth)acrylate esters of non-tertiary alkyl alcohols, the alkyl groups of which comprise from about 1 to about 18 carbon atoms, preferably about 4 to about 12 carbon atoms, and mixtures thereof. Examples of suitable (meth)acrylate monomers useful in the present invention include, but are not limited to, methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl acrylate, decyl acrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, hexyl acrylate, isoamyl acrylate, isodecyl acrylate, isodecyl methacrylate, isononyl acrylate, isooctyl acrylate, lauryl acrylate, 2-methylbutyl acrylate, 4-methyl-2-pentyl acrylate, ethoxyethoxyethyl acrylate, isobomyl acrylate, isobornyl methacrylate, 4-t-butylcyclohexyl methacrylate, cyclohexyl methacrylate, phenyl acrylate, phenyl methacrylate, 2-naphthyl acrylate, 2-naphthyl methacrylate, and mixtures thereof Particularly preferred are 2-ethylhexyl acrylate, isooctyl acrylate, lauryl acrylate, n-butyl acrylate, ethoxyethoxyethyl acrylate, and mixtures thereof
Examples of other ethylenically unsaturated monomers include, but are not limited to, vinyl esters (e.g., vinyl acetate, vinyl pivalate, and vinyl neononanoate); vinyl amides; N-vinyl lactams (e.g., N-vinyl pyrrolidone and N-vinyl caprolactam); (meth)acrylamides (e.g., N,N-dimethyl acrylamide, N,N-dimethyl methacrylamide, N,N-diethyl acrylamide, and N,N-diethyl methacrylamide); (meth)acrylonitrile; maleic anhydride; styrene and substituted styrene derivatives (e.g., alpha-methyl styrene); and mixtures thereof
Optional acidic monomers may also be used for preparation of the (meth)acrylate polymers. Useful acidic monomers include but are not limited to, those selected from ethylenically unsaturated carboxylic acids, ethylenically unsaturated sulfonic acids, ethylenically unsaturated phosphonic acids, and mixtures thereof Examples of such compounds include those selected from acrylic acid, methacrylic acid, itaconic acid, fumaric acid, crotonic acid, citraconic acid, maleic acid, beta-carboxyethyl acrylate, 2-sulfoethyl methacrylate, styrene sulfonic acid, 2-acrylamide-2-methylpropane sulfonic acid, vinyl phosphonic acid, and the like, and mixtures thereof
A suitable class of useful (meth)acrylate polymers is described in U.S. Pat. No. 4,554,324. This patent discloses reinforcement of conventional (meth)acrylate polymers by modification of the (meth)acrylate polymeric backbone by grafting reinforcing polymeric moieties onto the (meth)acrylate polymeric backbone. The reinforcing polymeric moieties may be grafted, for example, by in-situ polymerization of the reinforcing polymeric moieties in the presence of and onto reactive sites of the ungrafted (meth)acrylate polymer backbone, reacting prepolymerized polymeric moieties with reactive sites of the ungrafted (meth)acrylate polymer backbone, or by copolymerizing reinforcing polymeric compounds with monomer used to prepare the (meth)acrylate polymer backbone to form the (meth)acrylate polymer grafted with reinforcing polymeric moieties.
The reinforcing polymeric moieties in this embodiment generally have a Tg (glass transition temperature) of at least 20xc2x0 C. and a weight average molecular weight of at least 2,000. By contrast, the ungrafted (meth)acrylate polymer backbone generally has a Tg of less than about xe2x88x9220xc2x0 C., usually less than about xe2x88x9250xc2x0 C. in this embodiment. Preferred reinforcing polymeric moieties are those based on polymethylmethacrylate and polystyrene.
The Tg of a reinforcing polymeric moiety or ungrafted (meth)acrylate polymer backbone is measurable using Differential Scanning Calorimetry using second heat measurements at 10xc2x0 C. per minute.
Depending on the particular application, other suitable monomers, including diene monomers, may be copolymerized with the (meth)acrylate monomers when preparing the (meth)acrylate polymer. However, in one embodiment, the (meth)acrylate polymer of the invention is derived from essentially no diene monomers.
Propylene-Derived Polymer
Any suitable polymer can be used for the propylene-derived polymer. The propylene-derived polymers themselves, may or may not have PSA properties. Generally, the propylene-derived polymer is at least amorphous, preferably non-stereoregular. As such, the compositions of the invention are able to have enhanced pressure-sensitive adhesive properties, often without the need for using substantial amounts of additives, such as plasticizers or liquid oils.
The propylene-derived polymer is derived from at least propylene monomer. While other types of monomers may be used in their preparation, typically the propylene-derived polymer is derived from greater than 60 mole percent propylene monomers. Other monomers that can be copolymerized with the propylene monomer include, for example, alpha-olefin monomers (e.g., ethylene, 1-butene, 1-hexene, 1-heptene, 1-octene, 1-nonene, etc.).
It is preferred that the propylene-derived polymer contains a saturated hydrocarbon backbone. Accordingly, preferably the propylene-derived polymer is derived from essentially no diene monomers. Many compositions derived from diene monomers are relatively unstable over time, such as for example, when exposed to weathering or higher temperatures (e.g., when hot-melt processing).
Propylene-derived polymers of the invention are of high enough molecular weight that they do not act as a tackifier or plasticizer. Typically, the weight average molecular weight of the propylene-derived polymer is at least about 5,000 grams/mole. Preferably, the weight average molecular weight of the propylene-derived polymer is at least about 10,000 grams/mole, even more preferably at least about 15,000 grams/mole, and even more preferably at least about 20,000 grams/mole. Particularly useful are polymers with a weight average molecular weight of about 10,000-1,000,000 grams/mole, preferably about 20,000-200,000 grams/mole.
According to one aspect of the invention, the propylene-derived polymer is a copolymer derived from at least propylene and ethylene monomer. Any suitable amount of ethylene monomer may be used to prepare such propylene/ethylene-derived copolymers as long as the resulting copolymer is amorphous. Generally, however, the greater the proportion of ethylene monomer used, the more likely it is that the resulting copolymer will not be amorphous.
Particularly useful are the propylene/ethylene-derived copolymers with a glass transition temperature (Tg) of about xe2x88x9250xc2x0 C. to about 0xc2x0 C., preferably greater than xe2x88x9240xc2x0 C. to about 0xc2x0 C., and more preferably about xe2x88x9230xc2x0 C. to about 0C. Generally, when the Tg of the propylene/ethylene-derived copolymer is lower than xe2x88x9250xc2x0 C., it is because a larger proportion of ethylene monomer was used in preparation of the copolymer. While some such copolymers may be useful for certain embodiments of the invention, as discussed above, these polymers may not be amorphous. Furthermore, it is preferred that the Tg of the propylene/ethylene-derived copolymer is greater than about xe2x88x9250xc2x0 C. in order to reduce the necessity for adding a tackifier, or at least a large amount of tackifier, to the composition in order to obtain PSA properties for room temperature applications. The Tg of a polymer is measurable using Differential Scanning Calorimetry using second heat measurements at 10xc2x0 C. per minute.
Examples of propylene/ethylene-derived copolymers useful in the present invention include polymers commercially available from Eastman Chemical Co.; Kingsport, Tenn. under the EASTOFLEX tradename and polymers commercially available from The International Group; Wayne, PA under the KTAC tradename. Specific examples of suitable propylene/ethylene-derived copolymers from these companies are those with a Tg of about xe2x88x9233xc2x0 C. to about xe2x88x9223xc2x0 C., such as EASTOFLEX E1060, EASTOFLEX E1200, and KTAC 6013.
More generally, according to another embodiment of the invention, the propylene-derived polymer is a copolymer derived from at least propylene and one other alpha-olefin monomer, such as 1-hexene. A specific example of a commercially available propylene/hexene-derived copolymer is that sold under the trade designation, EASTOFLEX D127S, available from Eastman Chemical Co.; Kingsport, Tenn.
According to another aspect of the invention, the propylene-derived polymer is derived from essentially 100 percent by weight propylene monomers. Any suitable polypropylene can be used in accordance with this aspect of the invention.
When the propylene-derived polymer is derived from essentially 100 percent by weight propylene, the preferred Tg of these polymers is about xe2x88x9215xc2x0 C. to about 10xc2x0 C., more preferably about xe2x88x9210xc2x0 C. to about 5xc2x0 C. The use of at least one propylene-derived polymer having such a preferred Tg facilitates formation of a composition having PSA properties. Again, the Tg of a polymer is measurable using Differential Scanning Calorimetry using second heat measurements at 10xc2x0 C. per minute.
When higher molecular weight propylene-derived polymers, particularly polypropylene, are preferred, those polymers prepared using a metallocene catalyst, such as in PCT Publication No. WO 99/20,664, are particularly useful. Typically, polymers prepared using a metallocene catalyst (i.e., metallocene-generated polymers) have a weight average molecular weight of greater than about 70,000 grams/mole, which is typically higher than the molecular weight of many commercially available non-stereoregular propylene-derived polymers. A similar comparison applies when comparing melt viscosities of the polymers. Propylene-derived polymers prepared using metallocene catalysts may be preferred when PSA compositions having higher shear strength are desired in addition to improved peel adhesion properties. The higher molecular weight of the propylene-derived polymers prepared using a metallocene catalyst also enables them to be more usefully crosslinked, as compared to those propylene-derived polymers having lower molecular weights. This may be the case, when for example, the PSAs are to be used in a high performance application.
According to one embodiment of this aspect of the invention, the stereoregularity index (S.I.) of the propylene-derived polymer is about 1.0 to about 5.0. Preferably, when the propylene-derived polymer is amorphous, its S.I. is about 1.0 to about 1.05. Preferably, when the propylene-derived polymer is semi-syndiotactic, its stereoregularity index (S.I.) is about 1.1 to about 4.0.
As stated previously, however, one advantage of the present invention is that the blends are tailorable for a wide variety of applications. Higher molecular weight polymers may not always be preferred depending on the application. For example, lower molecular weight polymers may be preferred when using the PSA composition to form a fiber (e.g., melt-blown fiber). PSA blends of the invention may be advantageously used to prepare melt-blown microfiber webs, for example. Addition of a lower molecular weight polymer to a conventional polymer composition in accordance with the invention tends to lower the melt viscosity of the polymer composition at a given processing temperature. Therefore, the use of polymer blends of the invention may facilitate hot melt processing of fibers (e.g., microfibers) from PSA compositions at lower temperatures than those used to hot melt process fibers from conventional PSA compositions. Also, the use of polymer blends of the invention may facilitate a higher throughput of hot melt processed fibers at a given processing temperature.
The melt viscosity of the propylene-derived polymer can vary widely. Typically, however, the melt viscosity of the propylene-derived polymer is at least about 10 Poise when measured at 190xc2x0 C. according to the Viscosity Test method in the Examples section, infra.
In one embodiment, particularly those embodiments where the propylene-derived polymer is derived from essentially 100 percent by weight propylene, melt viscosity of the propylene-derived polymer is greater than about 500 Poise, more preferably greater than about 750 Poise, when measured at 190xc2x0 C. according to the Viscosity Test method in the Examples section, infra. In a further embodiment according to this aspect of the invention, the melt viscosity of the propylene-derived polymer is greater than about 2,500 Poise when measured at 190xc2x0 C. according to the Viscosity Test method. In still a further embodiment of this aspect of the invention, the melt viscosity of the propylene-derived polymer is greater than about 10,000 Poise when measured at 190xc2x0 C. according to the Viscosity Test method.
Generally, the higher the melt viscosity of the propylene-derived polymer, the more likely it is that the resulting composition will have a higher shear strength in conjunction with improved peel adhesion properties. This is particularly beneficial when preparing PSA compositions of the invention for high performance applications.
Optional Tackifier
Tackifiers of the invention have a weight average molecular weight of less than about 10,000 grams/mole and may be a in a solid or liquid state. The compositions of the invention may include a tackifier, where necessary to impart the desired PSA properties. Those of ordinary skill in the art recognize that a wide variety of tackifier are suitable for this purpose. The amount of tackifier used is readily appreciated by one of ordinary skill in the art.
Preparation of Blends
PSA compositions of the invention include at least one (meth)acrylate polymer and at least one propylene-derived polymer. Other additives (e.g., antioxidants, crosslinking additives, fillers, and ultraviolet stabilizers) may also be added to the PSA compositions, depending on the desired application and as well known to one of ordinary skill in the art.
Each of the (meth)acrylate polymer and propylene-derived polymer components of the blend is preferably present in an amount of about 5 weight % to about 95 weight % based on total weight of the blend. More preferably, each of the components is present in an amount of at least about 10 weight % based on total weight of the blend. Typically, however, the (meth)acrylate polymer component is present in a major portion and the propylene-derived polymer component is present in a minor portion based on total weight of the two components. This ratio of components contributes to obtainment of compositions having adequate adhesion to both relatively high surface energy substrates and low surface energy substrates. It has also been found that this ratio facilitates formation of compositions having useful shear strengths by, for example, facilitating crosslinking of the pressure sensitive adhesive composition.
No matter what proportion of the total blend each of the polymeric components comprises, the propylene-derived polymer component is present in at least about 15 weight % based on total weight of the (meth)acrylate polymer and propylene-derived polymer components. Below this amount, significant improvements in peel adhesion to at least one of high or low surface energy substrates is not as readily obtainable. Preferably, the propylene-derived polymer component is present in at least about 20 weight %, more preferably about 20 weight % to about 50 weight %, based on total weight of the (meth)acrylate polymer and propylene-derived polymer components.
According to a farther embodiment of the invention, the (meth)acrylate polymer component is present in at least about 15 weight % based on total weight of the (meth)acrylate polymer and propylene-derived polymer components. Preferably, the (meth)acrylate polymer component is present in at least about 20 weight %, more preferably about 20 weight % to about 50 weight %, based on total weight of the (meth)acrylate polymer and propylene-derived polymer components.
Blending of the polymers is done by any method that results in a substantially homogeneous distribution of the polymers. The polymers can be blended using several methods. In particular, the polymers can be blended by melt blending, solvent blending, or any suitable physical means.
For example, the polymers can be melt blended by a method as described by Guerin et al. in U.S. Pat. No. 4,152,189. That is, all solvent (if used) is removed from each polymer by heating to a temperature of about 150xc2x0 C. to about 175xc2x0 C. at a pressure of about 5 Torr to about 10 Torr. Then, the polymers are weighed into a vessel in the desired proportions. The blend is then formed by heating the contents of the vessel to about 175xc2x0 C., while stirring.
Although melt blending is preferred, the PSA blends of the present invention can also be processed using solvent blending. In that case, the polymers in the blend should be substantially soluble in the solvents used.
Physical blending devices that provide dispersive mixing, distributive mixing, or a combination of dispersive and distributive mixing are useful in preparing homogenous blends. Both batch and continuous methods of physical blending can be used. Examples of batch methods include those methods using BRABENDER (e.g., a BRABENDER PREP CENTER, available from C. W. Brabender Instruments, Inc.; South Hackensack, N.J.) or BANBURY internal mixing and roll milling (available from FARREL COMPANY; Ansonia, Conn.) equipment. Examples of continuous methods include single screw extruding, twin screw extruding, disk extruding, reciprocating single screw extruding, and pin barrel single screw extruding.
Applications
The PSA compositions of the present invention can be readily applied to a substrate. For example, the PSA composition can be applied to sheeting products (e.g., decorative, reflective, and graphical), labelstock, and tape backings. The substrate can be any suitable type of material depending on the desired application. Typically, the substrate comprises a nonwoven, paper, polymeric film (e.g., polypropylene (e.g., biaxially oriented polypropylene (BOPP)), polyethylene, polyurea, or polyester (e.g., polyethylene terephthalate (PET)), or release liner (e.g., siliconized liner).
PSA compositions according to the present invention can be utilized to form tape, for example. The PSA is applied to at least one side of the backing. The PSA may then be crosslinked to further improve the shear strength of the PSA. Any suitable crosslinking method (e.g., exposure to radiation, such as actinic (e.g., ultraviolet or electron beam) or thermal radiation) or crosslinker additive (e.g., including photoactivated and thermally activated curatives) may be utilized.
When double-sided tapes are formed, the PSA is applied onto at least a portion of both sides of the backing. Alternatively, a release material (e.g., low adhesion backsize) can be applied to the opposite side of the backing, if desired. Advantageously, the PSA and/or release material, for example, can be coextruded with the film backing for ease of processing.
The PSA can be applied to a substrate using methods well known to one of ordinary skill in the art. For example, the PSA can be applied using melt extrusion techniques. The PSA composition can be applied by either continuous or batch processes. An example of a batch process is the placement of a portion of the PSA composition between a substrate to which the PSA is to be adhered and a surface capable of releasing the PSA to form a composite structure. The composite structure can then be compressed at a sufficient temperature and pressure to form a PSA layer of a desired thickness after cooling. Alternatively, the PSA composition can be compressed between two release surfaces and cooled to form, for example, a transfer tape.
Continuous forming methods include drawing the PSA composition out of a heated film die and subsequently contacting the drawn composition to a moving plastic web or other suitable substrate. A related continuous forming method involves extruding the PSA composition and a coextruded release material and/or backing from a film die and cooling the layered product to form an adhesive tape. Other continuous forming methods involve directly contacting the PSA composition to a rapidly moving plastic web or other suitable preformed substrate. Using this method, the PSA composition is applied to the moving preformed web using a die having flexible die lips, such as a conventional film or sheeting die. After forming by any of these continuous methods, the films or layers can be solidified by quenching using both direct methods (e.g., chill rolls or water baths) and indirect methods (e.g., air or gas impingement). Hot melt processed fibers can also be prepared using another continuous forming method. Examples of this process can be found, for example, in PCT Publication No. WO 99/28,539.
Although coating out of solvent is not preferred, the PSA compositions can be coated using a solvent-based method. For example, the PSA composition can be coated by such methods as knife coating, roll coating, gravure coating, rod coating, curtain coating, and air knife coating. The coated solvent-based PSA composition is then dried to remove the solvent. Preferably, the applied solvent-based PSA composition is subjected to elevated temperatures, such as those supplied by an oven, to expedite drying.